These days most of us would probably not use SPM 8:2 as a traditional summing mixer. For that purpose its noise floor doesn’t match todays recording equipment. But it has a few tricks up it sleeve that makes it interesting as a true analog effect machine. Among them an adjustable transition time between program changes (soft VCA controlled EQ, effect, volume changes) and assignable LFO. It’s a nice utility to do automated control of analog audio from a sequencer.
I had the lid off mine to see if there are any op-amp’s I could easily replace to better the noise performance. Sadly I’ve not been able to track down schematics for my SPM 8-2, but a quick look at some of the datasheets for IC’s used in this mixer we learn each channel are based around the LM1035N, a dual DC operated Tone/Volume/Balance circuits and a SSM2024 quad VCA. Two TL084 quad J-Fet op-amps takes care of amplification, buffering and I/V convertion. In the master section we find more TL084s and SSM2024s.
I’m sure we could get less distortion than SSM2024's 0.3% THD with modern VCA’s from THAT. But cost and amount of work on the main board makes for special interest doing so. And the true limitation still remains with the noise of LT1035, listet somewhat optimistic at s/n ratio 80dB/0dB gain but more realistic s/n 64dB/-20dB gain.
TL084 in this low gain application are doubtfully the source of noise. Although we could replace them for a modern TL074 (18 nV/√Hz) for better on-paper spec. If any of the op-amp stages are used for DC control the TL084 where considered to be the better one of the two. That might also be true for I/V conversion from SSM2024 outputs. Although, a modern TL074 probably surpass any vintage TL084 offset spec. so it should be fine for that purpose also.
Alternative MC33079P (4.5 nV/√Hz) are popular replacement for TL084 in high gain circuits (which this is not) for its sonic behavior when pushed hard.
It seems SPM 8:2 even more than traditional mixers need optimized input trim to get noise under control, and leave it at that as it’s not worth upgrading IC chips inside this unit.
Demultiplexing with included Sample & Hold circuits for DC control are handled by multiple SSM2300 chips. Less clutter than traditional 4051’s and op-amps, but more difficult to source genuine parts if one of them goes bad.
A very simple modification could be to implement two filter stages to replace the to two large filter caps after the rectifier diodes. Maybe try some snipper caps on the diodes.
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